This proposal is focused on the molecular biology of a new G protein- coupled receptor that mediates the actions of the neuroendocrine peptide growth hormone-releasing hormone (GHRH) on its target cell, the growth hormone-secreting pituitary somatotrope. Using PCR-based cloning strategies, candidate GHRH receptor cDNA clones from the pituitary gland were isolated and subsequently shown to encode a protein with seven potential membrane-spanning domains that binds to GHRH with high affinity and specificity and activates intracellular cAMP production when expressed in human or animal cell lines. The presence of a missense mutation in this GHRH receptor in the dwarf little mouse suggests an important role for GHRH and its receptor in the regulation of body growth in vertebrates. The objectives of this proposal are to identify domains within the GHRH receptor protein important for its biochemical functions in ligand binding and signal transduction, to investigate the regulation of GHRH receptor synthesis in the pituitary gland, and to establish the role of the GHRH receptor or related receptors in mediating the actions of GHRH in non- pituitary endocrine tissues. The proposed experiments are divided into four interrelated specific aims as follows: l. To elucidate the role of the cysteine-rich amino-terminal domain of the pituitary GHRH receptor in ligand recognition. 2. To determine the functional defect in the GHRH receptor protein of little mice and to assess the consequences of expressing GHRH receptor transgenes in these dwarf mice. 3. To establish the structure of the rat GHRH receptor gene and examine its expression and regulation in pituitary somatotroph cells. 4. To investigate heterogeneity in the GHRH receptor and to ascertain whether additional related receptors are expressed in GHRH-responsive non- pituitary tissues. Because of the important role played by GHRH in modulating both the proliferation and the secretory activity of pituitary somatotroph cells, an understanding of how this hormone transmits a biological signal through its receptor will be likely have direct applications to human health in the areas of pituitary tumorigenesis and disorders of growth hormone secretion and growth.